1. Technical Field
This invention relates to semiconductor devices and, more particularly, to a method of doping Group III-V compound semiconductor crystal layers with p-type impurities.
2. Discussion
Group III-V, especially Group III nitride compound semiconductor materials such as gallium nitride (GaN), gallium aluminum nitride (GaAlN), indium gallium nitride (InGaN) and indium aluminum gallium nitride (InAlGaN) are promising as materials for light-emitting devices. Various techniques are known for growing Group III nitride semiconductor crystal layers such as the metalorganic chemical vapor deposition (MOCVD) method, the molecular beam epitaxy method, and the hydride vapor phase epitaxy method. The MOCVD method is also sometimes referred to as the OMVPE process (organometallic vapor-phase epitaxy). These methods are discussed in the technical literature and need not be described in detail herein. It should be noted, however, that all of these methods use separate sources for active nitrogen and for the Group III elements that react to form a Group III nitride layer on a suitable substrate.
There is great interest in GaN growth for electronic devices such as light emitting diodes and laser diodes which require pn junctions necessitating the introduction of p-type and n-type impurities into GaN layers. N-type doping of GaN has not presented much of a problem. However, difficulties have been experienced in satisfactorily doping GaN with p-type impurities. Examples of prior art techniques for doping p-type semiconductor devices of this type is disclosed, for example, in U.S. Pat. No. 5,306,662 and the following papers: Maruska et al, "Violet Luminescence of Mg-doped GaN", Applied Physics Letters, Vol. 22, No. 6, 15 Mar. 1973; Amano et al, "P-type Conduction in Mg-Doped GaN Treated with Low-Energy Electron Beam Irradiation (LEEBI)", Japanese Journal of Applied Physics, Vol. 28, No. 12, December 1988, pp. L2112-L2114 and Nakamura et al, "Thermal Annealing Effects on P-Type Mg-Doped GaN Films", Japanese Journal of Applied Physics, Vol. 31 (1992) pp. L139-L142.
Unfortunately, it has been extremely difficult to obtain p-type GaN layers with a high p-type free carrier (hole) concentration, e.g., exceeding 1.times.10.sup.18 cm.sup.-3. The achievement of high p-type doping levels is hindered generally by several factors. The first factor is high acceptor ionization energy which results in incomplete activation of acceptors at normal working temperatures. The other factor is the interaction of acceptor impurities with material defects in Group III nitrides which results in a formation of deep levels in a forbidden gap. For instance, Zn, which is widely used in other III-V materials as a p-type dopant, is known to form preferentially deep levels in GaN resulting in high-resistivity material. Even the only currently known acceptor impurity successfully used for p-type doping of GaN-magnesium (Mg), was shown to form a sufficient amount of deep levels to be measured and identified. These levels may tentatively be described as being formed by Mg atoms residing on nitrogen sites (while shallow acceptors are formed by Mg atoms in the Group III sublattice). Magnesium also has an extremely high vapor pressure at growth temperatures commonly used for GaN growth. Thus, to achieve a sufficiently high Mg concentration, extremely high flows of the Mg source should be used. Unfortunately, it has been observed that very high Mg source concentrations at the growth interface can lead to morphology deterioration of the growing GaN film.
Therefore, it would be desirable to provide a commercially practical method for solving one or more of these difficulties in obtaining a semiconductor layer, particularly of a Group III nitride compound, with a high concentration of p-type impurities.